The photoelectrochemical oxidation and dissolution of silicon to form porous silicon is known. In this regard, it has been discovered that regular arrays of macropores with extremely large aspect ratios may be formed in n-type silicon substrates by photoelectrochemical anodic etching in hydrofluoric acid. (V. Lehman and H. Föll, Formation Mechanism and Properties of Electrochemically Etched Trenches in N-Type Silicon, J. Electrochem. Soc. 137, 653 (1990); U.S. Pat. No. 4,874,484 to Föll et al.). Lehman and Föll have explained the silicon pore formation phenomenon on the basis of a so-called “space charge region” model, which model assumes that a photogenerated current focuses electron holes (h+) on pore tips because of the bending of the space charge region around the tips. In this regard, Lehman and Föll have hypothesized that a silicon wafer, when irradiated with photons of certain wavelengths, will promote valence electrons of the silicon atoms to an excited state and in so doing generate electron (e−)—electron hole (h+) pairs. An induced electric field created by an applied voltage potential between the silicon wafer (described as the working electrode) and a counter platinum electrode generates the photo current. The geometry of the pore tips focuses the strength of the electric field so as to attract the population of electron holes (h+) to these locations thereby promoting dissolution of silicon into the hydrofluoric acid environment. The space charge region model seemingly has some merit for explaining the formation of macropores (i.e., average pore size>50 nm) in n-type silicon, especially the formation of regular pore arrays of acicular or columnar macropores. (M. Christopherson, J. Cartensen, and H. Föll, Macropore-Formation on Highly Doped N-Type Silicon, Phys. Stat. Sol. (a) 182(1), 45 (2000); V. Lehman, U, Grüning, Thin Solid Films, 297, 13–17 (1997)).
In general terms, macroporous silicon may be formed within defined areas of n-type silicon wafers by first applying a pattern to the frontside of each wafer (using standard photolithographic techniques) by means of a mask (e.g., photoresist overlaying a low temperature oxide (LTO) or silicon nitride layer) having an ordered array of square openings (e.g., 5 μm squares with an 8 μm pitch). The patterned wafer may then be alkaline etched to transform the ordered array of square openings in the mask into an ordered array of inverted pyramids, the tips of which act as electron hole (h+) focusing points (generally needed for subsequent silicon dissolution). The wafer may then be etched with hydrofluoric acid under an anodic bias with backside illumination in a specially configured photoelectrochemical etch cell apparatus. As is known in the art, most traditional photoelectrochemical etch cells include a frontside chamber filled with a hydrofluoric acid solution used for the silicon etching, and a backside chamber filled with either a concentrated H3PO4 or KCl solution used for establishing an ohmic electrical contact with the wafer (V. Lehman, Electrochemistry of Silicon, Wiley-VCH Verlag GmbH, 17–20 (2002)). In some photoelectrochemical etch cells, the solution filled backside chamber is replaced by simply highly doping the entire backside of the wafer, or by applying a transparent film of GaIn eutectic to the backside of the wafer.
In accordance with the space charge region model, electron holes (h+) generated by the backside illumination diffuse through the wafer and promote the dissolution of silicon mainly at the pore tips—thereby resulting in the formation of very high aspect ratio pores generally extending along the <100> direction (e.g., perpendicular to the front surface of the wafer). The physical characteristics associated with pore size, spacing, porosity, and specific surface area within the silicon wafer are generally determined by processing conditions such as, for example, selected mask pattern, hydrofluoric acid concentration, current density, bias potential, dopant type, dopant density, and crystal orientation.
A significant problem associated with existing macroporous silicon formation techniques, however, relates to the undercut etching that typically occurs underneath the frontside masked areas (refer generally to prior art FIG. 1B). In this regard, it has been reported that although macropore arrays with good quality may be obtained in the interior regions of the patterned masked area, “there will always be some macropore growth under the mask—in a rather irregular fashion.” (H. Föll, M. Christopherson, J. Cartensen, and G. Hasse, Formation and Application of Porous Silicon, Mat. Sci. Eng. R. 39 (4), 93–141 (2002)) (see also, M. Christopherson, P. Merz, J. Quenzer, J. Carstensen, and H. Föll, A New Method of Silicon Microstructuring with Electrochemical Etching, Phys. Stat. Sol. (a), 182 (1), 561 (2000) (discussing trench-formation at the nitride mask edge); A. Jaballah et al., Chemical Vapour Etching of Silicon and Porous Silicon: Silicon Solar Cells and Micromachining Applications, Phys. Stat. Sol. (a) 202 (8), 1606–10 (2005) (“[o]ne can notice a small lateral growth of porous structures at the edge of the holes [pores], which confirm silicon dissolution under the photoresist mask.”)). Indeed, existing techniques for making macroporous silicon have not successfully addressed the problem of undercut etching underneath the masked areas of the silicon substrate.
Accordingly, and in view the foregoing, there is still a need in the art for new and improved structures and related methods useful for making porous silicon in defined unmasked areas of a silicon substrate without significant undercut etching underneath the masked portions of the substrate. The present invention fulfills these needs and provides for further related advantages.